For a long time, lead brass has been used for the valves such as taps, ball valves and gate valves for water supply. Although the production cost of lead brass is relatively low and the valves assembled with the valves bodies produced therefrom can meet the use requirements, the lead can pollute the environment and is harmful to human health. Accordingly, its use has been increasingly restricted. If such valves are applied for the potable water supply systems, the release of lead into water will be in excess of the safety standard (for example, under NSF/ANSI Standard 61-2007-Drinking Water System Components, the lead into water should not exceed 5 μg/L, and the antimony into water should not exceed 0.6 μg/L).
At present, among all lead-free brass alloys, only the cuttingability of bismuth alloys are closest to that of lead alloys. However, there exist some drawbacks in the process of producing bismuth alloys, for example, poorer weldability, narrower temperature range for forging and slow increase or decrease in temperature required during the heat treatment of ingots or products. After assembling with the valve bodies which are forged with bismuth brass extruded bars supplied by many domestic and overseas copper manufacturers, most of valves will rupture after fuming with 14% ammonia for 24 hours because they can not eliminate the assembly stress by annealing.
The existing lead-free free-cutting antimony brass alloys have good cold and hot formability and superior corrosion resistance properties, but the release of antimony in the products prepared therefrom into water exceeds 0.6 μg/L via testing, and thus such products can not be used for the accessories in the potable water supply systems. Moreover, the valves produced therefrom tend to rupture without eliminating the assembly stress due to the stress corrosion.
Lead-free free-cutting silicon brass is also one of the research focuses in the field of the lead-free copper alloys. Currently researched and developed lead-free free-cutting silicon brasses are mainly high-copper and low-zinc deformation silicon brasses (the zinc content is about 20wt %), the stress corrosion resistance and dezincification corrosion resistance for such brasses are superior. The valves with a large torque of 100-130 N·m still do not occur stress corrosion cracking without eliminating assembly stress even if they are fumed with 14% ammonia for 24 hours. However, such values are lack of marketing competition due to high total production cost caused by high copper content.
The high zinc silicon brass alloys having good cuttability, castability, cold and hot formability and weldability, which are researched and developed by our company, have been applied to bath tap products in large scale and exported to European and American markets. Small scale valves sand casted by such alloys can pass the ammonia fuming test in which the valves are fumed with 14% ammonia for 24 hours without eliminating the assembly stress by annealing. However, when such alloys are used for larger scale valves with an assembly torque of 100-130 N·m, the valves tend to rupture due to the stress corrosion.